1. Field of the Invention
The present invention relates to improvements in a method of producing a subterranean structure having a ceiling, such as a subway tunnel, which comprises the steps of excavating a ditch or hole in the ground to a desired subterranean level, placing the ceiling directly on the excavated ground at this level, filling the hole in the ground, and driving an arched tunnel into the ground below the ceiling.
2. Description of the Related Art
Such a tunnel making method is known, for example, from German Pat. No. 2,829,712. It has the advantage of doing away with usual support structures and is, therefore, very economical and efficient. This is due to the fact that the tunnel ceiling is supported simply and directly on the existing ground at the bottom of the surface excavation, the longitudinally extending edges at respective sides of the ceiling lying on the existing ground and being supported thereby. However, the method disclosed in this patent has the disadvantage that a hole of excessive width must be excavated to enable the ceiling to be placed, which is due to the fact that, at both sides of the tunnel, sufficiently wide longitudinally extending strips must be laid bare to constitute supports for the longitudinally extending edges of the ceiling. Additionally, the tunnel ceiling itself must be of a width far in excess of the width required to cover the tunnel. Furthermore, experience has shown that a ceiling installed in this manner cannot satisfactorily prevent surrounding earth from crashing through the tunnel arch.